1. Field of the Invention
The present invention relates to a sensor control circuit embodied in the form of an integrated circuit (hereinbelow referred to as “IC”) on which a sensor such as an acceleration sensor is mounted, or to a sensor control circuit that controls for example an acceleration sensor. The present invention particularly relates to interruption output whereby interruption processing is performed by supplying for example a sensor measurement result or acceleration detection result to for example a central processing unit (hereinbelow called a “CPU”) of a host.
2. Description of the Related Art
Conventionally, acceleration sensors that detect acceleration are employed in various applications. For example, Japanese Patent Application Kokai (Laid-open) No. 2005-91219 discloses a device for detecting dropping of for example portable equipment. This device includes a sensor control circuit having an acceleration sensor.
The dropping detection device having a sensor control circuit disclosed in Japanese Patent Application Kokai No. 2005-91219 is provided in portable equipment such as a digital camera and exercises control such that the portable equipment is brought into a safe state by detecting the falling of the portable equipment and supplying a dropping detection signal to a control section of the portable equipment, before the portable equipment reaches the ground or collides on something. This dropping detection device includes for example: a piezo-resistance element type triaxial acceleration sensor that detects the acceleration in three axial directions; acceleration calculation means that calculates the acceleration using the detection result obtained from the acceleration sensor; and dropping determination means that, based on the calculation result from the acceleration calculation means, compares the weightless condition with a threshold value and determines that dropping is taking place when the weightless condition exceeds the threshold value. The dropping determination means supplies a dropping detection signal to the control section of the portable equipment. The acceleration calculation means and the dropping determination means may be implemented for example by program control of a host CPU.
The conventional method of supplying the measurement results of acceleration obtained by the acceleration sensor to the acceleration calculation means in the host CPU involves either amplifying the sensor output (i.e., an analogue detection signal) and supplying the resulting analogue value, or subjecting the analogue value to analogue/digital conversion (hereinbelow referred to as “A/D conversion”) and supplying the resulting digital value. However, in the case of supplying a pure analogue value or A/D converted value, the processing load on the host CPU is heavy. Thus, lightening of the load was sought. Accordingly, an IC was developed whereby all of the processing performed by the host CPU was carried out by a sensor control circuit, only the final conclusion being reported to the host CPU.
FIG. 2 of the accompanying drawings is a layout diagram showing a conventional sensor control circuit 10 embodied in the form of an IC.
The sensor control circuit 10 causes interrupt processing to be performed in a host CPU 20. The sensor control circuit 10 includes an acceleration sensor 11, amplifier circuit (hereinbelow referred to as “AMP”) 12, A/D conversion circuit 13, digital processing section 14, a single interrupt terminal (hereinbelow referred to as “INT terminal”) 15, and a serial interface (hereinbelow referred to as “serial IF”) 16. The digital processing section 14 performs for example acceleration calculation and dropping determination processing.
In the sensor control circuit 10, when the triaxial acceleration is detected by the acceleration sensor 11 and an analogue detection signal is generated, this analogue detection signal is amplified by the AMP 12, and then converted to a digital detection signal by the A/D conversion circuit 13, before being delivered to the digital processing section 14. The digital processing section 14 performs for example acceleration calculation and dropping determination processing using the digital detection signal that is applied thereto.
The data supplying scheme employed for this sensor control circuit 10 is that the acceleration detection condition and the internal processing condition (notification of interrupt) are reported to the host CPU 20 using the single INT terminal 15, and the measurement result is sent to the host CPU 20 using the serial IF 16.
The conventional sensor control circuit 10 shown in FIG. 2 has the following problems (a) and (b):
(a) There is only one INT terminal 15 for notification of the interrupt. In order to receive and process a plurality of reports on a plurality of conditions, therefore, the host CPU 20 must first of all perform factor analysis by reading the content of an interrupt request register, not shown, that is provided for example in the digital processing section 14. Consequently, even when a plurality of measurements are performed by the sensor control circuit 10, the host CPU 20 must give maximum priority to the interrupt request from the sensor control circuit 10 irrespective of whether its order of priority is in fact high or low.
(b) In the case of the sensor control circuit 10 being used for dropping detection, the detection result of acceleration 0 G (G represents the acceleration due to gravity and 0 G represents the weightless condition) and the detection result determined based on the threshold value for the acceleration are of a high degree of urgency since they are used for detection of dropping. Therefore, these detection results must be processed with maximum priority. However, if there are other competing interrupts, the above-mentioned detection result processing is delayed due to either failure to recognize the interrupt concerned (i.e., the interrupt needed for the detection result processing) or lowering of its order of priority.